A flood barrier across the mouth of Lake Pontchartrain — a 50-year-old idea to protect New Orleans and communities along the lake's north shore from storm surge during a tropical storm or hurricane — could cause millions of dollars in additional flooding damage to other parts of coastal Louisiana and Mississippi, according to a new study.
The reason is that water kept out of the lake by the barrier would instead flow onto areas north and south of it.
The proposed barrier, which is included in Louisiana’s master plan to rebuild and protect the coast, could cost St. Bernard Parish an average of $14 million a year in extra damage from severe storms, the study says. In Mississippi’s three coastal counties, it could cause an average of about $22 million in damage annually.
But those estimates are much smaller than the $1.4 billion in potential savings in flood damage to areas around the lake.
“We see huge benefit,” said Jordan Fischbach of the RAND Corp., which published the report. “But of course, it’s always a trade-off. The water needs to go somewhere.”
The latest version of Louisiana’s Coastal Master Plan calls for building flood gates at the Rigolets and Chef Menteur passes, which connect Lake Pontchartrain to Lake Borgne and the Gulf of Mexico.
The project, to be built within 30 years, would cost $2.4 billion in today’s dollars.
The master plan forecasts a wet future for Louisiana’s coastal communities because of rising sea levels, sinking coastal land and eroding wetlands. It proposes relying on pipelines, massive river diversions, levees and floodwalls for protection.
However, even if all the restoration projects work as they should, 2,800 square miles of the state’s coast could still disappear in the coming decades.
The new report analyzes the costs and benefits of putting barriers at the mouth of the lake, looking at communities from Ascension Parish to the Mississippi-Alabama line.
It was written by researchers with the Infrastructure Resilience and Environmental Policy Program of the RAND Corp., which is working with the state on its 50-year coastal rebuilding plan.
The barrier would include about a mile of earthen levee as high as U.S. 90 and gates at the two passes, each 2 feet high and 150 feet long. The gates would rise vertically out of the water and would allow some water to pass over them.
Communities on the north shore, including Covington, Mandeville and Slidell, would be the primary beneficiaries, although they would still be at risk from flooding by water already in the lake.
During Hurricane Isaac in 2012, storm surge from Lake Pontchartrain caused up to 9 feet of flooding along the north shore.
The idea of building a barrier at the mouth of Lake Pontchartrain goes back to the late 1960s, in the wake of Hurricane Betsy, but the U.S. Army Corps of Engineers rejected the idea.
It was reintroduced as a potential project in the 2012 version of the state's master plan. The latest version of the plan, released in January, proposes a scaled-down barrier to be built by 2047.
Previous versions of the barrier considered a levee as high as 24.5 feet along U.S. 90, but the state Coastal Protection and Restoration Authority and RAND decided that would cause too much flooding in Mississippi and St. Bernard Parish.
“We’re really trying to balance water flowing into the lake and water being pushed off into areas adjacent to the lake in a storm,” Fischbach said.
The barrier would reduce flood depths in most areas around the lake by 1 to 2 feet during a so-called 100-year storm.
St. Tammany Parish would benefit the most. About $620 million a year could be saved in potential damage.
St. John the Baptist Parish would also benefit greatly, saving $209 million annually. Orleans Parish would save $142 million in damage and Jefferson Parish $150 million.
The damage estimates for Mississippi were calculated by averaging the risk posed by storms ranging from a 10-year event, which has a 10 percent chance of occurring in any year, to a 2,000-year event, which has less than a 0.05 percent chance of happening.
St. Bernard’s damage projections account only for more severe — and rarer — storms.
Fischbach noted that Mississippi’s annual flood damage is already projected to be $1.6 billion without the Lake Pontchartrain flood gates.
Still, some officials are not happy that their communities could face more flooding because of the barrier.
“Any deviance to the normal flow of water — yes, I have concerns,” said Rupert Lacy, the emergency management director of Harrison County. “The water’s got to to go someplace. You stop it from moving west, it pushes it north or south.”
Hancock County, just across the Louisiana border, could incur an average of $14 million more flooding damage each year due to the barrier.
The annual damage estimates are $5 million in Harrison County, where Gulfport and Biloxi are located, and $2 million in Jackson County, which borders Alabama.
It’s far from certain that the surge barrier will actually be built. The entire master plan is estimated to cost more than $90 billion, and funding hasn’t been secured for projects years down the road.
“There’s a lot of work that’s going to need to go into the details of this thing,” said Bren Haase, chief of the Coastal Protection and Restoration Authority’s research division.